Field of the Invention
The invention concerns a method to determine a magnetic resonance system control sequence to generate an image series of a defined image region of an examination subject. The magnetic resonance system control sequence includes a multichannel pulse train with multiple individual RF pulse trains to be emitted in parallel by the magnetic resonance system via different independent radio-frequency transmission channels of a transmission device. The multichannel pulse train includes an excitation pulse to excite the image region, as well as a subsequent number of refocusing pulses in order to respectively excite an echo signal to acquire raw data for an image of the image series. Furthermore, the invention concerns a method to generate an image series of a defined image region using such a magnetic resonance system control sequence, and a method to determine structural data of an examination subject based on such an image series. Moreover, the invention concerns a corresponding control sequence determination device to determine an aforementioned magnetic resonance system control sequence for an image series, as well as a structural data determination device to determine structural data of an examination subject on the basis of a corresponding image series, and a magnetic resonance system comprising such a control sequence determination device and/or structural data determination device.
Description of the Prior Art
In a magnetic resonance system, the body to be examined is typically exposed to a relatively high basic magnetic field (known as the B0 field)—for example of 1.5 Tesla, 3 Tesla or 7 Tesla—with the use of a basic field magnet system. A magnetic field gradient is additionally applied by a gradient system. Radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmission system by suitable antenna devices, which cause nuclear spins of defined atoms excited to resonance by this radio-frequency field to be flipped (deflected) by a defined flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency magnetic field is also designated as a B1 field. This radio-frequency excitation, or the resulting flip angle distribution, is also designated in the following as a nuclear magnetization, or “magnetization” for short. Upon the relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated that are received by means of suitable reception antennas and then are processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner.
It is conventional to operate the transmission antennas of the radio-frequency transmission system in a “homogeneous mode”. For this purpose, a single temporal RF signal is passed to all components of the transmission antennas) so that an optimally homogeneous B1 field is present in the region of the examination subject from which data are to be acquired. In newer magnetic resonance systems, it has by now become possible to populate individual transmission channels with individual RF signals adapted to the imaging. For this purpose, a multichannel pulse train is emitted that (as described above) includes multiple individual radio-frequency pulse trains that can be emitted in parallel via the different independent radio-frequency transmission channels. In the measurement space (and consequently also in the patient), the previously homogeneous excitation can thereby be replaced by an excitation that, in principle, is arbitrarily shaped. Such a multichannel pulse train—also designated as a “pTX pulse” due to the parallel emission of the individual pulses—can include excitation, refocusing and/or inversion pulses.
The matching RF pulse train for the individual channels is typically designated with a suitable optimization program so that the local target magnetization distribution is achieved. For example, a method to develop such a multichannel pulse train in parallel excitation methods is described by W. Grissom et al.: “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation”, Mag. Res. Med. 56, 620-629, 2006. For a specific measurement, the different multichannel pulse trains that are to be emitted via the different transmission channels of the transmission device, the gradient pulse train (with matching x-, y- and z-gradient pulses) to be emitted in coordination therewith, and additional control specifications are defined in a measurement protocol, which is created in advance and (for example) can be retrieved from a memory for a specific measurement and be modified on site by the operator as necessary. During the measurement, the control of the magnetic resonance system then takes place wholly automatically on the basis of this measurement protocol, with the control device of the magnetic resonance system reading out and executing the commands from the measurement protocol.
In particular for an examination of moving subject structures (for example of the heart of a patient), for the determination of specific information it is helpful to acquire not only individual images but also an entire image series of the structure of interest in order to thus detect structural changes over time, for example how a heart chamber deforms during the movement of the heart. For this purpose, it is known (for example within the scope of a classical EPI sequence) to emit an excitation pulse in order to initially selectively excite the tissue in the desired image region (for example a slice through the heart) and then to emit a subsequent number of refocusing pulses, so an echo signal is induced in the tissue of the excited image region, which echo signal can be acquired as raw data for an image of the image series. This means that an excitation of the desired slice is implemented once and a single MR image of the slice can be acquired for each echo signal, respectively at an interval of approximately 30 ms, in a subsequent readout train, until the transverse decay of the magnetization (which takes approximately 1 s). This has also previously been implemented with a classical excitation without the use of pTX pulses. In order to increase the ability to detect the movement of the structure within the individual images and to allow an automatic image processing, wide-area sinc saturation pulses have conventionally been radiated in order to achieve as a “tagging” before the excitation. The sinc saturation pulses cause a lateral, periodic, stripe-shaped signal suppression within the image region. The images of the image series are then populated with a stripe pattern or grid pattern, and the stripe structure or grid structure deforms together with the movement of the structure. These global stripes or grids overlaying the entire individual MR images have an extraordinarily interfering effect on a normal viewing of the images, and in addition valuable image information that (for example) is located under a stripe may possibly not be detected.